A comprehensive analysis on cores

thin sections and logging data is combined with seismic facies

seismic attri-butes and palaeo geomorphology to identify platform margin and trace the distribution and migration of platform-margin facies belt in southeastern Tarim Basin during different deposition periods of the Cambrian and the Middle and Lower Ordovician.The rock types of platform-margin facies belt are mainly calcarenite

bioclastic limestone

oolitic limestone and micritic limestone.On seismic reflection profiles

platform margin beach facies present as chaotic or blank reflections

and the reef-beach complexes show mound reflection exteriorly and chaotic reflection or blank reflection interiorly.Both the buildup features with increasing stratum thickness caused by rapid growth of large reef-beach complexes

and the steep slope or distal steepening features caused by decreasing stratigraphic thickness in the basin

are indicators of platform margins.The method that uses natural gamma ray spectrometry logging data to identify carbonate lithology and sedimentary energy is combined with the data of organic carbon

carbon and oxygen isotopes

to map the relative sea level changes and their controlling effect upon platform margin migration in different sedimentary periods in southeastern Tarim Basin.Slope-basin facies with under-developed platform margin had dominated the Early Cambrian.With sea level continuously falling during the Middle to Late Cambrian

restricted platform facies started to take shape with edges migrating basin ward.In the Early Ordovician

the platform margin narrowed due to sea level rising during the Penglai Formation deposition and widened as the sea level falling again during the Lower Yingshan Formation deposition.The upper Yingshan Formation experienced rapid transgression which narrowed the platform edge and the lower Yingshanzu Formation went through falling sea level which turned the platform into open ones and widened the platform margin.However

the platform was again submerged as sea level rapidly rising after the deposition of the Yijianfang Formation.